38 research outputs found

    Chemical abrasion applied to LA-ICP-MS U-Pb zircon geochronology

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    Zircon (ZrSiO4) is the most commonly used mineral in U-Pb geochronology. Although it has proven to be a robust chronometer, it can suffer from Pb-loss or elevated common Pb, both of which impede precision and accuracy of age determinations. Chemical abrasion of zircon involves thermal annealing followed by relatively low temperature partial dissolution in HF acid. It was specifically developed to minimize or eliminate the effects of Pb-loss prior to analysis using Thermal Ionization Mass Spectrometry (TIMS). Here we test the application of chemical abrasion to Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) by analyzing zircons from both untreated and chemically abraded samples. Rates of ablation for high alpha-dose non-treated zircons are up to 25% faster than chemically abraded equivalents. Ablation of 91500 zircon reference material demonstrates a ca. 3% greater down-hole fractionation of Pb-206/U-238 for non-treated zircons. These disparities necessitate using chemical abrasion for both primary reference material and unknowns to avoid applying an incorrect laser induced fractionation correction. All treated samples display a marked increase in the degree of concordance and/or lowering of common Pb, thereby illustrating the effectiveness of chemical abrasion to LA-ICP-MS U-Pb zircon geochronology

    Making and breaking Columbia (Nuna): formation of a critical metal province? A North Atlantic view

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    The Palaeoproterozoic supercontinent of Columbia (Nuna), assembled at 2.1-1.8 Ga, included almost all of the EarthÔÇÖs cratonic blocks (Zhao et al. 2004). Following assembly of the cratons along collisional orogens, accretionary tectonomagmatic belts developed along the supercontinent margins. The margins of the Archaean craton in Greenland are marked by the Paleoproterozoic Nagssugtoqidian orogen to the north, and Ketilidian accretionary belt to the south. In Scandinavia, the Svecofennian orogen and the Lapland-Kola belt are of similar age. Between these two areas lies the Lewisian Complex of NW Scotland, comprising Archaean crust reworked in the Palaeoproterozoi

    The North Pennine batholith (Weardale Granite) of northern England new data on its age and form

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    A three-dimensional gravity model of the North Pennine batholith is interpreted to indicate that it comprises five contiguous steep-sided plutons. The North Pennine batholith was previously referred to as the Weardale Granite, but this term is now restricted to the largest, most central pluton, which was proved by the Rookhope Borehole in 1961. The surrounding plutons are the Tynehead and Scordale plutons in the west, and the Rowlands Gill and Cornsay plutons in the east. A new U-Pb zircon age of 399.3 ? 0.7 Ma demonstrates an unequivocal Early Devonian (Emsian) age of emplacement for the Weardale Pluton. The similarity of the forms of the four unsampled plutons to the body beneath Rookhope, and to other Devonian granites in Northern England, strongly suggests that they are also of Devonian age. Seismic reflection evidence supports a contrast between relatively deep-seated Devonian granites and tabular Ordovician granites in northern England. The most conspicuous magnetic anomalies over the North Pennine batholith are associated with the Early Permian Great Whin Sill, but there is also evidence of magnetized basement rocks or denser magnetic intrusive phases on the flanks of the non-magnetic, low density plutons. A long-wavelength magnetic low can be explained in part by the granite puncturing a deep magnetic basement, although the demagnetized zone may extend beneath the batholith. A spatial correlation between the vein systems in the Northern Pennine Orefield and the form of the batholith suggests that the granite masses played a significant role in focussing mineralizing fluids into the overlying Carboniferous rocks. The highest temperature mineralization occurs over the Weardale and Tynehead plutons, where there is also the clearest evidence for the channelling of mineralizing fluids through the batholith. The other plutons are generally associated with a lower intensity of mineralization, which occurs on their flanks rather than above their roofs. The Rookhope and Tynehead areas may therefore mark the location of convective `chimneys? which were important in focussing the fluid circulation responsible for the mineralization. The coincidence of the `chimney? zones with the minimum of the long wavelength magnetic low suggests that the hydrothermal system may also have played a role in modifying crustal magnetization

    A Toba-scale eruption in the Early Miocene: the Semilir eruption, East Java, Indonesia

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    The Indonesian archipelago is well known for volcanic activity and has been the location of three catastrophic eruptions in the last million years: Krakatau, Tambora and Toba. However, there are no reports of large magnitude eruptions during the earlier Cenozoic despite a long volcanic record in Indonesia during subduction of Indian Ocean lithosphere since the Eocene. Here we report an Early Miocene major eruption, the Semilir eruption, in south Java, the main phase of which occurred at 20.7 ? 0.02 Ma. This major volcanic eruption appears similar in scale, but not in type, to the 74 ka Toba event. Its products can be identified elsewhere in Java and are likely to have been distributed widely in SE Asia and adjacent oceans. The Semilir eruption could have triggered a climate response, but cannot yet be linked with certainty to Early Miocene climatic events such as glaciations

    Detrital zircon provenance and Ordovician terrane amalgamation, western Ireland

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    Detrital zircon analysis of sandstones interbedded with c. 464 Ma ignimbrites in the lower Mweelrea Formation of the South Mayo Trough, western Ireland, suggests Ordovician source-rock provenance that corresponds to two distinct volcanic-arc phases on the Laurentian margin. East-derived sandstones contain a suite of zircons with a mean age of c. 487 Ma that suggests derivation from the Cambrian to early Ordovician Baie Verte Oceanic Tract arcÔÇôophiolite complex, locally represented by the Lough Nafooey arc rocks and the Clew Bay Complex. Zircons from south-derived sandstones within the Bunnacunneen conglomerate fan have average ages of c. 467ÔÇô474 Ma, and correspond to the Notre Dame arc and locally the Connemara metagabbro and orthogneiss suite. Granite clasts in the Bunnacunneen conglomerate are similar to the Connemara orthogneiss suite, in terms of both their geochemistry and their age (c. 471 Ma). The southerly derived sedimentary strata also include Archaean and Proterozoic zircon age spectra consistent with a Dalradian source. A southern provenance from the Notre Dame arc and Dalradian rocks suggests that the Connemara terrane lay to the south of the South Mayo Trough during middle Llanvirn times, from at least 464 Ma

    AERYN: A simple standalone application for visualizing and enhancing elemental maps

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    International audienceInterpretation of high spatial resolution elemental mineral maps can be hindered by high frequency fluctuations, as well as by strong naturally-occurring or analytically-induced variations. We have developed a new standalone program named AERYN (Aspect Enhancement by Removing Yielded Noise) to produce more reliable element distribution maps from previously reduced geochemical data. The program is Matlab-based, designed with a graphic user interface and is capable of rapidly generating elemental maps from data acquired by a range of analytical techniques. A visual interface aids selection of appropriate outlier rejection and drift-correction parameters, thereby facilitating recognition of subtle elemental fluctuations which may otherwise be obscured. Examples of use are provided for quantitative trace element maps acquired using both laser ablation (LA-) ICP-MS and electron probe microanalysis (EPMA) of the cold-water coral Lophelia pertusa. We demonstrate how AERYN allows recognition of high frequency elemental fluctuations, including those which occur perpendicular to the maximum concentration gradient. Such data treatment compliments commonly used processing methods to provide greater flexibility and control in producing elemental maps from micro-analytical techniques

    G├╝ney Menderes MasifiÔÇÖnde Sinorojenik Eosen l├Âkogranit magmatizmas─▒ ve tektonik ├Ânemi

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    G├╝ney Menderes MasifiÔÇÖnde ├╝st d├╝zeylerin g├╝neye hareket etti─či normal bile┼čenli G├╝ney Menderes makaslama zonunun (GMMZ) taban blo─čunda yer alan ortognayslar ile onlar─▒ yap─▒sal olarak ├╝zerleyen metasedimanlar l├Âkogranit k├╝tleleri taraf─▒ndan kesilirler. Bu l├Âkogranit k├╝tlelerinden birisi ge├ž Orta Eosen (U-Pb zirkon ya┼č─▒) ya┼č─▒ndad─▒r. Bu veri GMMZÔÇÖndaki sinorojenik normal hareketin ba┼člama ya┼č─▒ olarak yorumlanm─▒┼čt─▒r. En ya┼čl─▒ uyumsuz sedimanlar masifin erken MiyosenÔÇÖde y├╝zeye ├ž─▒kt─▒─č─▒n─▒ g├Âstermektedir. Ortognasylar ├╝stten normal bile┼čenli GMMS, alttan ise bir bindirme fay─▒ (B├╝y├╝k Menderes bindirme fay─▒, BMBF) ile s─▒n─▒rlan─▒rlar; bu kaya├žlar ├╝st d├╝zeylerin KKDÔÇÖya hareket etti─či s─▒k─▒┼čmal─▒ ├╝st d├╝zeylerin GGBÔÇÖya hareketini veren geni┼člemeli fabrikler olmak ├╝zere iki farkl─▒ yap─▒ sunarlar. Bu yap─▒lar─▒n da─č─▒l─▒m─▒ ve ili┼čkileri ile ArAr mika ya┼člar─▒ iki farkl─▒ y├Âne hareket veren fabriklerin mekan ve zamanda birlikte geli┼čtiklerini ├Ânermektedir. Dolay─▒s─▒yla, yap─▒sal ve jeokronolojik veriler ─▒┼č─▒─č─▒nda: (i) normal bile┼čenli GMMZÔÇÖnun yeniden hareket kazanm─▒┼č bir bindirme fay─▒, (ii) ortognayslar ise yeniden mobilize olmu┼č Prekambriyen granitlerinden olu┼čan ve kuzeye do─čru hareket eden bir ekstr├╝zyon kamas─▒ ┼čeklinde yorumlanm─▒┼čt─▒r. Bu modelde, ortognayslar bir gnays domunun y├╝zeydeki ├žekirde─čini temsil ederler.The mylonitic orthogneisses and structurally overlying metasediments in the southern Menderes Massif are both intruded by leucogranites in the lower plate of a regional ductile top-SSW southern Menderes shear zone (SMSZ). One of these leucogranite bodies yielded a late middle Eocene U-Pb zircon age. This age is interpreted to mark the initiation of synorogenic normalsense shearing along the SMSZ. The oldest unconformable sediments suggest that the massif was exhumed by the earliest Miocene. The orthogneisses occur between normal-sense SMSZ above and a thrust fault (B├╝y├╝k Menderes thrust fault, BMTF), below. The ortogneisses display two distinct fabrics as top-NNE contractional and locally overprinting top-SSW extensional structures. The distribution of two sets of fabrics and the Ar-Ar mica ages further propose that top-SSW normal-sense shearing was partly coeval with the top-NNE thrusting. The structural and geochronologic data is interpreted to suggest that (i) normal-sense SMSZ is as a reactivated thrust fault and (ii) the orthogneisses form a northward extruding wedge of remobilized Precambrian granite(s). In this model, the orthogneisses represent an exposed core of a gneiss dome
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